H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

Normally when I fly, on my EFIS I have just the hottest EGT temp displayed up top which is what I refer to 90% of time.Today I happened to enable more detailed view of all 4 EGT temps on my EFIS and to my surprise there was a huge difference between 1/2 and 3/4.At power settings more than 5100 or so the difference between 1/2 and 3/4 EGTs was around 150+F and growing which , as far as I could find out, it is way to much. When dropping to around 5000-4970 all 4 EGT readings were within 20-30 F , which I believe is normal.

It is very repeatable , as soon as I advance power past 5200, they start instantly diverging and when I pull back all readings all goes back to normal. Everything runs fine - no undue roughness or anything I could complain about , all other temps (CHT, OIL) seem to be normal, except these EGT readings

I had my carbs balanced about 60 hours back, around Sept 2017 or so ..so I guess am about halfway thru to my next annual.From what I can tell , the most likely culprit in this case would be out of synch carbs .. any other ideas ?

The RH carb drives cylinders 1 and 3. The left one drives 2 and 4. So if this were a carb synch issue, the odds would be the same, and differ from the evens (which would also match each other). But what you report is a difference between the front two and the back two cylinders. A few theories:

(1) uneven airflow under the cowling, so the front cylinders and the rear cylinders cool differently (but I would expect this to show up in the CHTs rather than the EGTs)

(2) different exhaust pipe lengths front and back (in my installation, 1 and 2 are long pipes, 3 and 4 are short ones) could have dictated different placement of the EGT probes

(3) EGT probes are miswired to the MFD or EMS, so they don't display in the proper order. (You can test this with a heat gun to the exhast pipes, one at a time, to see which line lengthens)

(4) you misspoke, and it is indeed 1 and 3 that differ from 2 and 4 (in which case, one carb is set richer than the other).

Let us know what you find with further testing.

The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.Prof H Paul ShuchPhD CFII DPE LSRM-A/GL/WS/PPC iRMTAvSport LLC, KLHVfly@AvSport.orgAvSport.org facebook.com/SportFlyingSportPilotExaminer.US

I just looked closely at your photos. You can clearly rule out my Option 4. I think 3 is most likely.

The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.Prof H Paul ShuchPhD CFII DPE LSRM-A/GL/WS/PPC iRMTAvSport LLC, KLHVfly@AvSport.orgAvSport.org facebook.com/SportFlyingSportPilotExaminer.US

The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.Prof H Paul ShuchPhD CFII DPE LSRM-A/GL/WS/PPC iRMTAvSport LLC, KLHVfly@AvSport.orgAvSport.org facebook.com/SportFlyingSportPilotExaminer.US

I will test the miswiring theory tonight - btw ... I noticed that the front exhaust pipes sensors are located on the pipe bend while the back are not.I am wondering if this difference has always been there I just never noticed ...

Interestingly, on the top picture you can see what appear to be yellow peak marks for the 3/4 readouts suggesting that these never went past 1300 F ( not sure if these are peak marks though - my TrueTrak EFIS is no longer being made and the docs I have for it are very sparse )

Warmi wrote: I noticed that the front exhaust pipes sensors are located on the pipe bend while the back are not.

Same as my setup (and the basis of my Theory #2).

The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.Prof H Paul ShuchPhD CFII DPE LSRM-A/GL/WS/PPC iRMTAvSport LLC, KLHVfly@AvSport.orgAvSport.org facebook.com/SportFlyingSportPilotExaminer.US

drseti wrote:The RH carb drives cylinders 1 and 3. The left one drives 2 and 4. So if this were a carb synch issue, the odds would be the same, and differ from the evens (which would also match each other). But what you report is a difference between the front two and the back two cylinders. A few theories:

(1) uneven airflow under the cowling, so the front cylinders and the rear cylinders cool differently (but I would expect this to show up in the CHTs rather than the EGTs)

(2) different exhaust pipe lengths front and back (in my installation, 1 and 2 are long pipes, 3 and 4 are short ones) could have dictated different placement of the EGT probes

(3) EGT probes are miswired to the MFD or EMS, so they don't display in the proper order. (You can test this with a heat gun to the exhast pipes, one at a time, to see which line lengthens)

(4) you misspoke, and it is indeed 1 and 3 that differ from 2 and 4 (in which case, one carb is set richer than the other).

Also, the placement of the EGT sensor probes. As per Rotax Installation Manual, "Readings of EGT taken approx. 100 mm (3.93 in) from exhaust flange connections."

If they are appreciably different, at locations along the pipe, it would not be surprising, to find different readings between them.

Dr Seti was right - this is not carbs , the cold EGTs are connected to back cylinders - 3/4 track each other very close ( the difference is maybe 10-30 degrees ) , same with the front ones - so the large difference is between front and back cylinders.The back pipes are significantly longer as well but the placement of EGT probes seem to be according to Rotax specs - of course the front ones are placed on the bent - I am not sure if it affects their temperature.

The only other thing I can think of is that recently I took off the cabin heater muff which is located right behind back cylinders and blocks large portion of the radiator - it is recommended to take it off for the summer ops to improve cooling.

Everything else , CHT and Oil are perfectly within specs and the engine runs fine - I will be syncing carbs soon and will have mechanic take a closer look at this and see if he can spot anything.

What’s interesting is that I would think 1 & 2 would run cooler given they are up front close to those large inlet holes on the front of the cowling. I have not looked close at the Sting cowling but it would seem you could almost see them and the exhaust looking in the front therefore would run cooler from the ram air coming in.

My 2007 Rotax only has two EGT’s and I have one probe on #2 (front) and one probe on #3 (rear). I understand that Rotax now only recommends measures the rear EGT (3&4) when only two EGTs are installed. I want to move my front probe to the rear but you know what they say, “if it ain’t broke.......”.

The thing is that both cylinder heads do run equally cool ( or hot ) - it is just EGTs that different between back and front.

Unfortunately my EFIS does not record any engine parameters for permanent storage so I can't really tell if this is a new development or it has been like that since I bought the plane last year. Over last year I did occasionally enable the detailed view with all 4 EGTs but that was mostly just a quick look at these values during stable cruise and at 5100-5200 where both readings tend to be within 100 F of each other. It is during full power application (takeoff , climb ) and ( interestingly ) during descent on idle that they diverge - when idle on the ground or when in stable cruise at around 5200 , both sets stay close to each other.

PS.Shawn. Is your front EGT probe attached at the pipe bent ? Cause I am starting to think that the difference between front and back sets is related to their different shape.

Being out of sync does not mean the carb itself is out, but the cables may be binding (throttle linkage) at certain rpms or a carb piston binding. Putting a set of gauges on to just look at lower rpm carb sync may not show this because it shows up at rpms above our normal static sync range. You need to make sure both carbs and throttle setup are doing the same at your problem rpm range. If the EGT's read the same most of the time and only off at a specific rpm then it most likely isn't the probes. It is more likely something affecting the fuel flow which changes the temp reading.

We will be syncing carbs soon but what , as dr seti suggested , points to potentially some other culprit is the fact that the difference is only between front and back EGTs - if it were related to carbs we would see difference between 1 and 3 or 2 and 4 ... but in my case both 1/2 are within 10-20 F of each other as are 3/4 - it is as if back EGTs were being cooled at different rate than the front ones.

Look at your air intake. Which cyl. are further away? Air imbalance can and does cause some inequity. That's why you may see carbon black plugs from idling on the front two.This said it can still come from other sources like plugs and plug gaps and these aren't the only two items.